Thursday, August 30, 2007

Rare bird news

An eventful few weeks for bird records in North America:

The long-anticipated first nesting record of Lesser Black-backed Gull in North America (even though it hybridized with a Herring Gull) - on Appledore Island, Maine.

Green Violetear reaches Maine - the farthest northeast record to date. 19 Aug 2007 on Mount Desert Island; photo here

First Fan-tailed Warbler in Texas, in Pine Canyon, Big Bend NP and still present 30 Aug 2007. One of few records in the US.

First Great Knot in West Virginia (and eastern North America; and one of fewer than five records in North America outside Alaska) demonstrating once again that rare birds can turn up anywhere. You just have to get out there and look. 13 Aug 2007 at Winfield Locks and Dam, WV (photo links down, hopefully back soon - 7 Feb 2008).

First Townsend's (Newell's) Shearwater for mainland North America. Details here. Found on land (!) at night on 1 Aug 2007, San Diego County, CA
    • Closely related to the other small black-and-white shearwaters, the AOU currently considers this a single species: Townsend's Shearwater (Puffinus auricularis), with two subspecies:
      • P. a. auricularis (commonly known as Townsend's) breeding on the Islas Revillagigedos off western Mexico.
      • P. a. newelli (commonly known as Newell's) and breeding on Kauai, Hawaii.
    • The two differ slightly in appearance (plumage and structure), habitat at sea, and breeding range and season. But identification and separation from Manx and other potential confusion species at sea is extremely difficult. Given trends in seabird taxonomy it seems likely that these two will be split into two full species, but neither had previously been documented within the birder's North America.
Tantalizing first report of Tristram's Storm-Petrel in North America - seen briefly but unfortunately not photographed from a pelagic birding trip off southern California on 21 July 2007.

The third verifiable North American record of Little Shearwater, and the first since the late 1800s, when beached specimens were found in Nova Scotia and South Carolina. The bird was seen and well-photographed during a pelagic birding trip south of Nantucket, MA on 25 Aug 2007. Photos here, in fact quite possibly the best photos ever taken of this species at sea.
  • taxonomy is confusing, since the AOU still maintains a widespread and variable species called Little Shearwater, but most authorities have split the North Atlantic breeders from populations in the rest of the world. The new species, called Macaronesian Shearwater, is comprised of two subspecies which also seem likely to be split into two species:
    • P. b. baroli - nesting in the Canaries, Azores, and Madeira and the source of all three North American records
    • P. b. boydi - nesting in the Cape Verdes
The totally unexpected first North American record of Brown Hawk-Owl - 27 Aug on St. Paul Island, Alaska (hiding among the crab pots, as many forest birds do on treeless St. Paul Island) photo and details here

Meanwhile, at Gambell on St. Lawrence Island, AK, Brown Shrike, Pechora Pipit, Willow Warbler and Dusky Warbler have all been seen in the past week. Check out updates here.

Finally, a Jabiru in Isola, Mississippi, reported 24-27 Aug 2007 (and possibly seen two weeks earlier as well) feeding at catfish farms with Wood Storks and Great Egrets. Photo and details here. This is the first for Mississippi, and one of fewer than ten records in the US of this spectacular bird.

Wednesday, August 29, 2007

Judging size of birds

Size judgment is one of the constant quandaries of bird identification - critically important but fraught with error. In a recent online discussion about these photos of sandpipers in flight, I was intrigued by the question of how I, and others, "just knew" that these birds were too small to be Knot or Pectoral and must be Least or Semipalmated. There are no reference points in the picture, so the cues for judging size must come from the birds themselves, and proportions of different body parts must hold the key.

Mystery sandpiper photo by Corey Finger, who relates the whole story here

Now this photo (above) has been posted and discussed on ID-Frontiers, with no consensus on the identification, but with Least and Sharp-tailed - two species of very different sizes - both getting votes. Again I was intrigued by the question of size judgment. I traced bird outlines from two photographs to compare body size and proportions.

These outlines (below) show two species, different sizes in life but adjusted here so that the wing length and body size are about the same.
In this demonstration, the most striking difference is the relative size of the head, making the upper bird look big-headed while the lower bird looks small-headed. There also appears to be a difference in the back end, at least in these two images, but I can cover up that part of the image and still get a strong size impression from just the front half of these two birds.

The upper sketch is traced from a Least Sandpiper photograph, the lower is traced from a Sharp-tailed Sandpiper photograph (both photos in Crossley, Karlson, and O'Brien, The Shorebird Guide).

Does this actually help identify the mystery sandpiper in the photo above? I think so. To me the mystery bird's head size matches the Least Sandpiper better than the Sharp-tailed. And I think head size is also the key to judging the size of the flying sandpipers in the earlier mystery photos. It may not be definitive, but it's another point to add to the mix.

This is not to say that larger birds of all species look relatively small-headed (Cooper's vs. Sharp-shinned Hawk is an obvious exception) but whenever we are judging size in the field or from photos it's worth pausing to think about what goes into that judgment. Most often it does not involve actual direct comparison of sizes, and I suspect that experienced observers subconsciously rely on body proportions in a lot of cases.

Thursday, August 23, 2007

Olive-sided Flycatcher - white patches and age in fall

Several recent Massachusetts reports of Olive-sided Flycatchers seem to have used the presence of white patches on the sides of the back to age the birds as juveniles. This is incorrect and unfortunately the source of the error seems to be an ambiguous entry in the Sibley Guide - sorry.

Both adults and juveniles can show the white patches, it has nothing to do with age, the patches appear and disappear depending on the bird's posture. When I painted the images I arbitrarily chose to show the white patches on the juvenile, but then failed to make it clear that adults are equally likely to show them. So please update your guides with the information that the white patches, when visible, are a distinctive feature of both adults and juveniles of the species.

If you want to try to age Olive-sided Flycatchers in the fall it should be pretty straightforward (given a very good view), since they don't molt until they are on the wintering grounds. This means that adults on fall migration are wearing feathers that were grown almost a year ago, and look worn with indistinct grayish wingbars, while the juveniles are in fresh plumage just a month or two old, and look smoother and cleaner with fresh primaries and more distinct buffy wingbars.

Good Birding,
David Sibley

Sandwich Tern - bill color of juveniles

-- updated Sep 27, 2008

This photo by Greg Lavaty of Texas, generated a lot of discussion on the Texas Birds listserv recently, and brings up some interesting points and questions.

First, this is definitely a juvenile Sandwich Tern, and the bill color is pretty typical of a recently fledged bird. Here are some photos of similar birds in Texas by Martin Reid. Few birders have the opportunity to see birds at this age, and I've only seen it a few times on the Texas coast in summer, so this is a very instructive photo. In the Sibley Guide I illustrated the juvenile with a bill pattern similar to Greg Lavaty's photo (orange along the cutting edges of the bill), and stated "bill dull orange before fledging, quickly becomes blackish with pale tip". This is not strictly accurate on a couple of points, as Ted Eubanks reports on Texbirds [19 Aug 2007 in Galveston, TX]: "Today I saw young Sandwich Terns with bills from completely yellow-orange to black with no yellow tip. Of course, I found every permutation in between."

Sandwich Terns through their first year typically have a black bill with a much-reduced pale tip compared to adults (contrary to the juvenile illustrated in the Sibley Guide), and their bills can look all black. I did use this species for a comparison image in Birding Basics, where I illustrated the bill pattern of adult vs juvenile correctly.

More importantly, some young Sandwich Terns show a yellow-orange bill with little or no black for a substantial period of time after fledging, and such birds are apparently not that unusual. To the unaware, this can cause some real confusion. Here's a photo of a mostly orange-billed juvenile Sandwich Tern by Greg Lavaty.

From the comments: observations by Joseph Kennedy at Galveston, Texas in August 2007:
Newly flying chicks have bills that are yellowish or maybe yellow-orange, very different from the orange color of the royal tern chicks. As they age the yellow fills in with streaks of tan and brownish from the outside in which becomes black.... This pattern appears to be random which can result in upper and lower beak differences or base and tip differences.

Last to change is the interior of the beak and the edge of the beak which is
still yellow when the outer beak is all dark. At the same time the beak is
growing longer. Originally the beak structure appears more like a
gull-billed tern and as the youngster ages the beak grows at the tip until
it reaches the long skinny beak of the adult.
Distinguishing these orange-billed juveniles from Royal and Elegant Terns must rely on size and shape, the paler gray upperparts of Sandwich, and at least on some birds (fide Ted Eubanks) the more spotted pattern of the back and scapulars compared to juvenile Royal (but compare variation in upperparts pattern of juvenile Sandwich from unmarked to heavily marked, as in other terns). Mostly it should just be a matter of being aware of the possibility and recognizing that juveniles are this variable.
  • Key point: Royal and Elegant Terns never show any black on the bill. So as soon as these juvenile Sandwich start to show black on the bill it should be easy to rule out other species. [Birds with mixed orange and black bills could also be hybrid Elegant x Sandwich Terns (which are very rare, but recorded in Texas and Florida), or 'Cayenne' Tern, the South American subspecies of Sandwich Tern (very rare visitor recorded in NC and NY]
Questions that follow from this very interesting observation are:
  • Could these birds' bill color be confused with Royal or Elegant Tern, or is the color different somehow? Ted Eubanks reports that the color can be just as bright yellow-orange as Royal Tern, but Joseph Kennedy says Royal is noticeably more orange (and of course some juvenile Royal Terns have very pale and drab yellowish bills). It does seem that most young Sandwich soon develop at least some dark smudges that make the bill look "dirty" or drab.
  • What percentage of birds show different variations, especially how many have completely yellow-orange bills with no black? Reports so far suggest this is a small percentage. Greg Lavaty from Texas on 26 Aug 2007 reports "the great majority of them (the young ones anyway) had what looked like all black beaks or black with only the slightest yellow tip."
  • How long can they retain orange color with or without black smudges? i.e. at what date is it safe to identify an orange-billed tern as "not-Sandwich". [From photos it appears that all develop at least some black on the bill very soon after fledging - July and August. And Martin Reid's photo from September 20th, and photos of a similar bird from Florida on October 19th by K. Dean Edwards indicate that at least some juveniles retain orange on the bill through October]
  • Could this be a regional variation? It seems unlikely that this is a local regional variation limited to Texas, but it would be great if observers from Florida to the Carolinas could check to see if a similar proportion of juvenile Sandwich Terns there show yellow or orange bills after fledging. This color is not normal in Sandwich Terns of Britain and Europe, where subspecies sandvicensis occurs (North American birds are subspecies acuflavida). Klaus Malling Olsen (Terns of Europe and North America, 1995) highlights a single British record of a juvenile with all yellow bill. So this certainly seems to be a difference between European and American subspecies. The South American subspecies eurygnatha - Cayenne Tern - has a yellow-orange bill as an adult, so one would expect a yellow-orange bill to be the norm for juveniles there.
Thanks to Ted Eubanks, Greg Lavaty, and Joseph Kennedy for comments and photos.

If you live where Sandwich Terns can be seen now, and/or have an answer or insight, feel free to post comments here, or to email me directly.
Good Birding,
David Sibley